1. Field of the Invention
The present invention relates to a gas detecting and monitoring device for detecting and monitoring the presence of inflammable or poisonous gas such as methane gas, hydrogen gas, ammonia gas, LPG or the like.
2. Description of the Prior Art
The gas detecting and monitoring device of this kind, which has been heretofore used, includes an indicating instrument portion arranged at a central monitoring position and gas detecting portions arranged at gas detecting positions at a distance such as few tens meter to 2 kilometer from said central monitoring position, said indicating instrument portion and said gas detecting portions being connected by transmission cables. The construction of the conventional gas detecting and monitoring device of this kind is schematically shown in FIG. 1. As schematically shown in FIG. 1, the gas detecting portion 10 comprises a wheatstone bridge which includes two sides consisting of a detecting element S.B and a compensating element R.B connected in series and other sides consisting of resistors R.sub.1, R.sub.2 and R.sub.3 connected in series. A power source E is connected through a constant-current circuit CR to one ends of said bridge circuit. The potential difference e between the other ends a and b of the bridge circuit is fed through a transmission cable 30 to an indicating instrument portion 20. The detecting element S.B consists of a coil of fine wire of platinum or the like, on which the material responsive to the gas to be detected, for example oxidizing catalyst, is adhered and sintered in the form of beads. The compensating element R.B consists of a coil of same platinum wire as that of the detecting element S.B. on which ceramic material is adhered and sintered, so that the compensating element R.B holds same temperature rise and same resistance value as those of the detecting element S.B in the normal air, and also these elements are formed in same shape. However, the compensating element is different from the detecting element in the point that the former is not responsive to the gas to be detected. In operation of this gas detecting and monitoring device, the current fed to the detecting element S.B and the compensating element R.B is automatically controlled to an optimum value through the constant-current circuit CR and then held at a constant current value, and when the power fed to the detecting element S.B and the compensating element R.B becomes equal to the thermal loss emanating from these elements to the surrounding atmosphere, the temperature of these elements becomes constant and under such condition the potential difference between a and b is made to be zero (by adjusting the variable resistor R.sub.2), so that the indicator of the indiating instrument portion 20 indicates zero value. Under such circumstances, if the methane gas, for example, exists and comes into contact with the detecting element S.B, it burns owing to the reaction of the gas with the catalyst, thereby generating heat, and the temperature of the heat thus generated adds to the temperature of the preheated platinum coil, so that the resistance value of the platinum coil increases in accordance with the temperature coefficient thereof. Accordingly, a potential difference between a and b of the bridge circuit appears and said potential difference is indicated on the indicator of the indicating instrument portion 20, thereby detecting the existence of the methane gas. In this case, this potential difference is proportional to the concentration of the existing methane gas.
In the conventional device as described above, the change of the resistance value depending upon the temperature change of the platinum wire which constitutes the elements S.B and R.B is linearly stable, as shown by curve A shown in FIG. 2, under usual conditions. Accordingly, the potential difference appearing between a and b of the bridge circuit is always held at a constant value when the gas concentration is zero, and therefore this bridge circuit is functionally satisfactory. However, the temperatures of the detecting element S.B and the compensating element R.B themselves (the temperatures owing to generation of heat) usually vary, depending upon the current value fed thereto and the resistance values thereof, and it is a usual practice to select such current value that the temperature of the detecting element becomes maximum at a predetermined room temperature (for example 20.degree. C.) and to hold the current value at a predetermined value by means of the constant-current circuit CR. For example, 130-300 mA current is fed to the platinum wire to heat the detecting element to the temperature between 300.degree. and 500.degree. C., and thus it is used under highly sensitive condition. Consequently, it has such disadvantage that the sensitivity and the indicated value of the detecting and monitoring device vary depending upon the variation of the ambient temperature.
The temperature change of the detecting element itself is considered to be produced on the following grounds.
(1) Change of the characteristics of the detecting element itself due to ambient temperature change:
In the open air the temperature varies, for example, in the range between -40.degree. C. (at night and cold time) and +80.degree. C. (at midsummer and daytime), and in the room the temperature varies, for example, in the range between -10.degree. C. (in factory or loading station and at night time) and +50.degree. C. (at working time in daytime). The detecting portion of the device is subjected to the influence of such temperature variation of the outside factor. That is, the temperature of the detecting element itself varies, owing to such temperature variation of the outside factor.
(2) Change of the characteristics of the parts due to ambient temperature change:
The resistance value of the platinum wire, the resistance value of the resistor, the drift of 1C and transistor in the constant-current circuit, the resistance value of the transmission cable connecting the detecting portion and the indicating instrument portion and the like are subjected to the influence of the ambient temperature change.
In general, a detecting element is heated by a constant current, as described above. Consequently, as the ambient temperature becomes higher, the resistance value of the element increases in accordance with the temperature coefficient of platinum, thereby causing a further rise of temperature, and as the ambient temperature becomes lower, the resistance value decreases, thereby causing a further lowering of temperature. In any case, the temperature departs from its optimum value, with the result that the sensitivity and the value of indication of the gas detecting and monitoring device are changed. The relation between such gas sensitivity and the temperature can be represented by the relation between the a-b potential difference of the bridge circuit and the temperature, for example, as shown by the curve B in FIG. 3. Also the relation between this gas sensitivity and the temperature can be represented by the ambient temperature and the indication error as seen on the indicator of the indicating instrument portion, for example, as shown by the curve C in FIG. 4. It will be clear from these curves B and C that even if the allowable range of error as .+-.3%, the conventional device produces an error beyond such allowable range, and therefore it cannot fully realize its primary purpose of detecting the leakage of inflammable or poisonous gas and foretelling and preventing the occurrence of accidents.
The compensation for such ambient temperature change might be obtained by connecting a thermal responsive semiconductors or the like to the elements of the bridge circuit. However, there is wide variation in characteristics of thermal responsive semiconductors, and therefore this method of compensation needs correction of characteristics of the thermal responsive semiconductors, that is troublesome, and may give rise to the lowering of the sensitivity of the detecting element. Accordingly, this method of compensation cannot be said to be a best method.
It is an object of the present invention to provide a gas detecting and monitoring device which eliminates the disadvantages as described above and which is not susceptible to the influences of the ambient temperature changes and can effect accurate gas detection as all time.